When a square can is double seamed, because the can has a corner seamed portion and a linear portion, it is difficult to perform seaming by rotating the can, at variance with the case of seaming a round can, and seaming is generally performed by revolving a first seaming roll and a second seaming roll, while controlling the trajectories thereof with a model cam having formed therein a cam groove having a shape similar to that of the can, in a state where a can body and a lid are clamped and fixed by a seaming chuck and a lifter (see the patent document 1). In such seaming of a square can, flange portions of the can body and can end are subjected to bending in the linear portion, but in the corner seamed portion, shrinking processing (that is, drawing) is conducted together with bending processing because seaming is accompanied by diameter reduction of the flange. Therefore, in the corner seamed portion, the sheet thickness increases due to metal flow caused by shrinking, and the portion that is not absorbed by sheet thickness increase remains as wrinkles or the flange width increases. This phenomenon becomes more prominent as the drawing ratio of the corner seamed portion increases. In the case of seaming a round can, drawing is also performed and similar phenomenon is observed, but because the curvature radius of the seamed portion in the round can is large, the drawing ratio is small and good seaming can be performed practically without the occurrence of wrinkles or flange elongation. As a result, few problems are associated with degraded sealing ability. However, in the case of square cans, the curvature radius of the corner seamed portion is much less than the round can diameter. Therefore, the drawing ratio obviously increases, wrinkles or flange elongation easily occur in the corner seamed portion, sealing ability in this portion deteriorates, and the sealing ability is inferior to that of round cans. Examples of means for resolving such problems inherent to square cans are suggested in the patent documents 2 and 3, but a satisfactory solution for problems arising when a corner seamed portion with a small curvature radius is seamed is yet to be found.
For this reason, a square shape has been conventionally employed for large cans such as five-gallon cans with a comparatively large curvature radius of corner seamed portion, and small square cans have been used for storing the contents that does not require a comparatively high level of sealing, such as cakes which are non-liquid contents. Thus, generally, small square cans have not been used for applications requiring a high level of sealing, such as beverage cans. However, a specific feature of square cans is that no gaps appear between the cans when they are assembled and the accommodation efficiency thereof is much higher than that of round cans. With this feature in view, a demand has recently been created for small square cans for special applications with high sealing ability that can be filled with contents requiring high sealing ability. In order to increase further the accommodation efficiency, which is a specific feature of squire cans, it is necessary that practically no dead space appear when the cans are stacked in the longitudinal-lateral and up-down directions, and in order to satisfy this requirement it is necessary that the curvature radius of the corner seamed portion of the seamed portion be reduced to obtain a corner seamed portion that is very close to a right angle and that the lid seamed portion be reduced to a minimum. On the other hand, from the standpoint of increasing the contents filling efficiency related to the volume occupied by a can, a can shape is preferred in which the upper and lower panel portions of the can ends are positioned at a very small depth from the end surface of can body, that is, that the distance from the top portion of the can seamed portion to the deepest position corresponding to the inflection portion where transition is made to the lower inner wall of the seamed portion or panel surface of the can end (usually referred to as “countersink depth”) be small. However, these requirements are diametrically opposite to those relating to the increase in sealing ability and can be damaging factors from the standpoint of sealing ability. For this reason, square cans that demonstrate a high level of sealing that enables them to be filled with liquid contents, have a small curvature radius of corner seamed portions, and also have a small countersink depth have not yet been obtained.    [Patent Document 1] Japanese Patent Application Laid-open No. 51-104469    [Patent Document 2] Japanese Patent Application Laid-open No. 58-58950    [Patent Document 3] Japanese Examined Patent Application No. 02-62094